Optical networking has shown itself to be valuable in core transport networks, in large part due to the use of wavelength division multiplexing (WDM). More recently, WDM optical networking has also become important in the metro access arena. The D.A.R.P.A. Next Generation Internet (NGI) initiative, for example, is especially interested in investigating the issues surrounding transport of Internet Protocol (IP) traffic over such networks, and recent commercial vendor activity in this space is also evidence of a growing awareness of the need for flexible and high-capacity solutions.
The demands on metro networks are stringent, since the traffic is more diverse than traffic in the core, yet the system costs must be smaller. Ring architectures are generally preferred since they offer more cost-effective management and protection options, as evidenced by the performance and acceptance of SONET systems that have been used in metropolitan (metro) office rings. As in the core networks, WDM is expected to play an important part for several reasons. First, it allows existing fiber to be “mined” for more bandwidth capacity by using additional wavelengths of light. This prevents “fiber exhaust” on existing routes, defers the need to deploy more fibers, and permits more flexible transport solutions. Traditional SONET equipment, for example, could be used on each wavelength, forestalling the need to go to higher data rates in the hierarchy. Second, more sophisticated optical networking (i.e. more sophisticated than simply increased transport capacity) can be performed by utilizing the wavelengths as optical channels, which can be provisioned, added, dropped, routed, and managed as individual entities, independent of the data format they carry. A third, and corollary reason is that WDM allows service transparency, permitting new services with independent formats to be developed and distributed without hardware or facility changes. The extra dimension in wavelength also permits efficient and cost effective terminal solutions through using transparency to transport data in native format, rather than requiring conversions and multiplexing. Transparency (with regard to optical networking) signifies that the optical signals do not undergo optical-to-electrical-to-optical conversions as they traverse the network. Additional cost-effective properties include stability and passivity.